Pre-charging switch arrangements for direct-current circuits are used in particular in high-voltage direct-current circuits in which a load, such as a capacitor, is intended to be connected to a power source. A frequent application involves, for example, Battery Disconnecting Units (BDU) in electric or hybrid drives of vehicles. Such drives generally have high-voltage batteries which are connected to an inverter for providing the required alternating-current or three-phase current voltage to a vehicle drive unit. For reasons of safety, such devices have two controllable switches, one for each pole. These switches may, for example, be contactors or relays.
If the high direct-current voltage source is connected directly to the uncharged capacitor, short-circuit currents may briefly flow and may lead to damage to the components. Consequently, the capacitor is commonly pre-charged before it is completely connected to the direct-current voltage source. A pre-charging switch arrangement is connected parallel with one of the two controllable switches. In order to pre-charge the capacitor, the other controllable switch and a pre-charging switch are then closed. The capacitor is thereby charged by means of a pre-charging resistor. If the capacitor has reached a desired charge state, the controllable switch which is connected parallel with the pre-charging switch arrangement is also closed and the load is connected to the direct-current voltage source. The pre-charging switch can then be opened again.
In order to monitor the charge state of the capacitor, it is conventional to tap and evaluate a voltage of the capacitor across the opened controllable switch. This voltage can provide information relating to the charge state of the capacitor. This type of voltage measurement, however, requires complex isolation of the high-voltage lines from an electronic control system which may be present and which evaluates the voltage and produces control signals for switching the controllable switches. In addition, the galvanic separation between the two sides of the controllable switches is consequently not reliably ensured; in the prior art, auxiliary relays are often used for galvanic separation. Known solutions thus make the pre-charging switch arrangement complex and costly.